The quest for a device that a person could strap on his back, lift off from his backyard, and fly around started in the 1940s. One of the more popular ideas came from Seattle, Washington, inventor Horace Pentecost. He actually designed and built a small backpack with a coaxial rotor system powered by a 20-hp two-cylinder air-cooled engine. The assembly weighed about 90 lbs and had a 12-foot rotor diameter. An overhead cyclic control also controlled yaw with a twist grip and moved vertically to control collective pitch.

Dubbed the “Hoppi-Copter” it was plagued with several issues like getting away from the fuselage quickly in an emergency and using the pilot’s legs as landing gear (too unstable). To solve some of these problems Pentecost built a second model with a 40-hp engine and simple tripod assembly for landing gear. Even so, the tiny aircraft proved unstable and too difficult to fly. Thinking the machine also had a military appeal, he continued working on improving the design.

However, shortly into the test program a major problem arose. Early on when he had formed his company he had given 10 percent of it to his lawyer in exchange for setting up the corporation. Pentecost and his wife divorced giving her half of his 90-percent ownership. His ex-wife and lawyer soon got together and Pentecost was pushed out of the company he started. In 1956 the company was sold to a group of investors from Washington, D.C. Their idea was to raise enough capital to certify the tiny helicopter by selling 300,000 shares of the company at $1 each. Ultimately, they were unsuccessful and the original Hoppi-copter ended up at the Smithsonian National Air and Space Museum where it remains in storage or out on loan.

The dual ducted-fan concept was explored by SoloTrek, who had achieved more than 1 hr of flight time before getting DARPA as investors. Following this, in 2005, they “retired” their eXsokeleton Flying Vehicle (XFV). http://www.trekaero.com/Trek_VTOL_Vehicles.htm

Ducted fans are not helicopters. The distinction is subtle… but helicopters use the main rotor in its rotational path to create a disc which acts as a large circular airfoil. The top part of the disk has the low-pressure side, generating lift. A ducted-fan system has no airfoil (or “circular wing” if you prefer). It is merely a lifting-force device.

There are other differences. The ducted-fan devices being tested today and yesteryear (http://www.theregister.co.uk/2009/06/16/urban_aero_say_mule_test_imminent/) don’t use variable-pitch props (rotors) and are therefore unable to autorotate, nor to gain any advantage from forward motion. I would be remiss not to say that two small diameter fans generate less lift than a main rotor half their size (pi r squared).

Ehud
Tucson AZ

Alex Kovnat

DARPA (Defense Advanced Research Project Agency), whose basic mission is to think outside the box, isn’t working on a backpack helicopter but is working on something we might think is just as far out: A variant of the familiar High Mobility Multipurpose Wheeled Vehicle (HMMWV, “Hummer”) with a helicopter rotor on top so it can move 4 personnel over impassible land obstacles.

Said government agency did not exist until the 1957-1958 timeframe, when it was created by the Eisenhower administration in the wake of the Soviet Union orbiting Sputnik over our heads. One wonders if DARPA, had it existed in the 1955-56 timeframe, would have provided seed money and engineering talent for further development of the Hoppi-copter.

Roger Connor

The Model 100 Hoppi-Copter (the one pictured) is on a display loan from the National Air and Space Museum to the Pima Air and Space Museum. The backpack machine never actually flew. Please email me about the picture featured.